A method for slicing wafers can be carried out using a wire saw. The basic construction and the functioning of a wire saw are described, for example, in US 2002/0174861 A1 or US 2010/0089377. A suitable wire saw accordingly comprises at least two wire guide rolls around which a sawing wire is wound multiply. This gives rise to wire sections which are tensioned between two wire guide rolls and arranged in a parallel fashion and which form a wire web through which the workpiece is moved during the sawing operation. Wire sawing methods in which, instead, the wire web is moved through the workpiece are also known.
Appropriate workpieces include materials which are separated into wafers, in particular blocks composed of semiconductor material, from which semiconductor wafers are sliced.
The wire guide rolls have a coating having a specific thickness and having grooves that guide the wire sections. The region of the surface of the coating wears with the duration of the loading by the sawing operation. As long as the coating is still thick enough, the worn region of the surface of the coating can be removed by being ground away and the thinner coating regenerated in this way can continue to be used.
The sawing wire is wound from a supply spool onto a receiver spool during the sawing operation. In this case, the running direction of the sawing wire is usually changed cyclically, as a result of which more comprehensive utilization of the sawing wire is achieved.
Slicing the wafers uses abrasive grain that removes material from the workpiece in a cutting mode of operation. The abrasive grain can be fixedly bonded to the sawing wire. More often, use is instead made of a sawing slurry in which the abrasive grain is dispersed and which is fed to the wire web.
Semiconductor wafers which are produced in this way should have side surfaces that are as flat and plane-parallel as possible. In order that wafers having such a geometrical characteristic can arise, an axial relative movement between the workpiece and the wire sections, that is to say a relative movement parallel to the central axis of the workpiece, should be avoided during the sawing operation. If such a relative movement nevertheless takes place, wafers having a curved cross section arise. The degree of bending of the wafers is often specified by a characteristic value called warp.
As a cause of the occurrence the abovementioned relative movement, changes in the length of the workpiece and of the wire guide rolls are mentioned in US 2010/0089377 A1, said changes being attributed to changes in temperature and an associated thermal expansion or thermal contraction. In actual fact, frictional heat is generated particularly during the movement of the sawing wire around the wire guide rolls and when the sawing wire engages into the workpiece, and, in particular, the temperature of the workpiece, of the wire guide rolls and of the bearings of the wire guide rolls is changed as a result of heat transport.
US 2002/0174861 A1 describes a method which provides for regulating the temperature of the workpiece during the sawing operation.
US 2010/0089377 A1 proposes a method in which the displacement of the workpiece in an axial direction is measured and the axial displacement of the wire guide rolls is regulated in such a way that it corresponds to that measured. The regulation of the axial displacement of the wire guide rolls is effected, according to one example, by cooling water being conducted through the shafts of the wire guide rolls and by adaptation of the temperature and/or flow rate of the cooling water. Furthermore, it is proposed to record a curve showing the displacement of the workpiece as a function of the cutting depth, and to regulate the displacement of the wire guide rolls on the basis of the curve.